TNEF’s Enough will open, read, and allow access to any data contained within a winmail.dat attachment file that has been encountered in the iOS Mail app, the process is the same on any iPhone, iPad, or iPod touch.

1 Exit Mail app in iOS
2 Click here to get the App from the iOS App Store
3 Re-launch Mail in iOS and open an email containing the winmail.dat attachment file
4 Tap on the “winmail.dat” attachment file and choose “Copy to TNEF’s Enough”
5 Assuming the file is readable, TNEF’s Enough will open in iOS and show you a list of items contained within the winmail.dat attachment